A Koster-Slater approach to the problem of localized states at semiconductor interfaces has been developed. It allows us to relate the existence and/or the energy position of interface states to some essential bulk features of the constituent materials and some interface-bonding parameters. The condition for the existence of localized states and the relevance of the model will be discussed comparing the predictions entailed by the latter with the results of ab initio calculations on the Ge/GaAs (110) interface.